The research can be divided into three parts. The first is the analysis of the arrangement of the different classes of base-pair sequences in the chromomeres of Drosophila chromosomes through the isolation of homogeneous populations of hybrid DNA molecules between fragments of Drosophila chromosomal DNA and the lambda bacteriophage or plasmid DNAs. E. coli are infected with hybrid molecules which are constructed by enzymatic methods in vitro; clones of infected cells are then isolated which contain specific hybrids as autonomously replicating plasmids or as prophage; and the hybrids isolated from the clones by standard methods. The particular chromomere(s) associated with a specific hybrid can be determined by in situ hybridization to polytene chromosomes, and the sequential arrangements in the chromomeric DNA determined by techniques worked out for viral DNAs. The second project derives from our ability to isolate Thomas circles formed fragments of chromosomal DNA after exonuclease treatment. This is a powerful tool for the topographical analysis of proximate repetitive sequences in animal chromosomes. We are presently using this tool to determine the arrangement of moderately repetitive sequences in the Drosophila chromosomes. The last project is concerned with the regulation of replication origins in Drosophila chromosomal DNA. In the first phase we have determined the fork rates and the distribution of origins in the rapidly replicating clevage nuclei (S phase equals 3-4 min) and in cell cultures (S equals 600 min). A model derived from these results is presently under test.